1. Field of the Invention
The present invention relates to zoom lenses for use in still video cameras, video cameras, broadcasting cameras, or photographic cameras and, more particularly, to zoom lenses whose zoom ratio is, for example 3, whose F-number is 2.5 and whose maximum angle of field is increased to 60.degree. or wider, while still keeping high resolution.
2. Description of Related Art
The conventional zoom lenses for use in electronic cameras, video cameras, etc., are, in most cases, of the 4-unit type comprising, in order from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, the second and fourth lens units being moved to effect zooming.
This allows the first lens unit of large diameter to remain stationary. With the use of the lens units of relatively small diameters as the zoom section, therefore, a merit is produced that, when moving the variator, an electrically operated actuator to be used may be weak in driving torque. Also, if the fourth lens unit is made to serve as the focusing lens, there is another merit that one more actuator solely for focusing is not only unnecessary to use, but the minimum object distance can be shortened from that for the zoom lens whose first lens unit functions as the focusing lens without having to much increase the optically effective diameter of the first lens unit.
Such a zoom configuration is disclosed in, for example, Japanese Laid-Open Patent Applications No. Sho 62-24213, No. Sho 62-206516, No. Sho 62-215225, No. Sho 63-123009, No. Sho 64-68709, No. Hei 2-12118, etc.
The optical systems for the video camera may generally be said to have their required resolution at an upper limit, depending on the standards for television signals of NTSC, PAL or other systems, or the standards of the method of recording video signals.
For the electronic camera (e.g. digital camera) to be used as an image input device for the personal computer, however, there is no standard on which to determine such an upper limit. To such a digital camera, for the reasons mainly on the cost, it has been in many cases to apply the solid-state image pickup element that is used for the video camera. As the handling of images on the computer becomes an ever more common practice, there is a growing demand for employing a solid-state image pickup element having many more pixels. Particularly, as the degree of fineness of the output devices such as a monitor or printer improves and the speed of communication between the devices improves, the required resolution for the digital camera serving as an input device becomes ever higher.
On the contrary, most of the optical systems disclosed in the above-mentioned publications are of the form that only assures a resolution on the order of that for the conventional video camera. So, the form is not adequately suited to the zoom lens for the high resolution digital camera.